Thermosetting coating powders are dry, finely divided, free-flowing solid materials at room temperature. Coating powders find particular utility in industrial coating applications because they are readily applied to a variety of conductive substrates, they use very little or no organic solvents, and excess coating powders can be readily recycled.
One class of curable coating powders is ultraviolet (UV) radiation curable powders. UV curable powders have the ability to flow, cure, and produce smoother coatings at much lower temperatures than previously possible with traditional thermosetting chemistry. This is primarily due to the curing reaction being triggered by photoinitiated radiation rather than heat. Typically, UV curable powders are formulated from solid unsaturated base resins with low glass transition temperatures (Tg), such as unsaturated polyesters, unsaturated co-polymerizable cross linking resins such as vinyl ethers, photoinitiators, flow and leveling agents, performance-enhancing additives, and, optionally, pigments and fillers.
During coating operations, the parts are preferably preheated to drive out substrate volatiles, then UV curable powders are applied to a substrate, e.g. using electrostatic spray techniques. The coated substrate is then heated to fuse the powders into a smooth molten coating. Light, which cures and hardens the coating into a durable, smooth, attractive coating. However, because of the very rapid cure of UV curable coatings, it has been difficult to obtain a low gloss, smooth UV cured coating and the coatings so formed tend to have a relatively high gloss appearance. For reasons of aesthetic preference and commercial application, low gloss coatings are desirable for certain applications. Generally, gloss reduction can be obtained in traditional powder coatings through the introduction of matting agents, such as fillers or waxes, which rise to the surface during curing and cause matting through disruption of the surface of the coating. However, because UV curable powders cure so quickly, there is not adequate time for the fillers and waxes to flocculate to the surface, and they become trapped within the coating. There is reduction in flow in the coating but little matting takes place. Higher amounts of filler or waxes may be used, but this tends to cause the powders to block or cake during normal storage and/or produce coatings with severe orange peel, limiting the amount of gloss reduction that could be attained.
U.S. Pat. No. 6,348,242 discloses UV curable powder coatings containing crystalline components, wherein the heat fused powder is further heated to flow out the crystalline components, followed by cooling to recrystallize them to form a low gloss finish prior to UV curing the coating. However, reliance on crystalline resins to provide film continuity or appearance properties of the coatings adds costly recrystallization processing to the formation of the powder and leads to powders lacking in storage stability.
Accordingly, there remains a need for UV curable powders that form powder coatings with a low gloss. In accordance with the present invention, the present inventors have found powders that are resistant to blocking and that give surprisingly durable, low gloss, smooth UV cured powder coatings.